His research expertise includes climate change effects on respiration and carbon cycling, climatic adaptation in plant traits, plantation forestry, urban forestry and the biogeography of forest tree species. A goal of his research is to advance fundamental knowledge of plant and ecosystem responses to environmental change and provide science-based information to inform policy choices. Professor Tjoelker served as an advisor to the U.S. Secretary of Agriculture through the Forestry Research Advisory Council. He has authored more than 125 refereed journal articles and edited the book, The Biology and Ecology of Norway Spruce (Springer, 2007). His published work is among the most cited (top 1%) in the refereed literature (ISI, Essential Science Indicators). Professor Tjoelker was appointed to the prestigious ARC College of Experts commencing in 2019. The College plays a key role in identifying research excellence in the ARC National Competitive Grants Program, moderating external assessments and recommending applications for funding.